Phosphorus (P) is critical for crop production but also poses a threat to water quality in Ohio. Therefore, a better understanding of optimizing the P availability to crops while minimizing the potential of P to pollute water bodies is important.
Plant P Use
Plants require P to generate adenosine triphosphate (ATP), which is an energy currency. It is also part of deoxyribonucleic acid (DNA) and ribonucleic acid (RNA) that make genetic code and synthesize proteins. A deficiency of P also negatively affects fundamental processes such as photosynthesis, seed production, maturation, and root growth. A P-deficient plant is often stunted and can develop dark green or purple pigmentation on older (lower) leaves (Fig.1).
Soil P Pools
Phosphorus is less abundant in soil than nitrogen and potassium. The total P content in soil varies from 0.0005% to 0.15%, but most of this total P is not readily available to crops. Some of the P is in the organic form, which is derived from plant residues, manure, or other organic sources. In Ohio’s relatively young soils, most of the P is in the inorganic P form. Inorganic P is bound to primary and secondary minerals, and a proportion of it can be bound to clay. Organic and inorganic P forms are in equilibrium with soil-solution P that provides P to growing crops.
When an external organic P source such as crop residue is added to soil, it can be immobilized or mineralized based on the carbon-to-phosphorus (C to P) ratio. If the C to P ratio of organic matter is greater than 300 to 1, the P can be immobilized into organic form and become relatively unavailable for uptake by plants. If the C to P ratio of organic matter is less than 200 to 1, the P from organic matter is mineralized into plant-available P forms. Similarly, when P fertilizers are added to soil, they increase the soil-solution P concentration. However, a large amount of applied P ends up in relatively unavailable P forms. These transformations are generally governed by soil pH. In acid soils, P adsorbs or precipitates as iron or aluminum secondary minerals, oxides, or clay minerals. In neutral to calcareous soils, calcium or magnesium minerals, oxides, and carbonates precipitate and adsorb the P. The amount of P adsorbed or precipitated from applied P depends on mineral solubility and adsorption capacity. Nonetheless, irrespective of the extent of precipitation and adsorption, the P-fixation by biological and chemical process is important to consider in order to optimize P fertilizer use efficiency (Fig. 2).
Plant P Uptake
A plant’s P uptake depends on the P availability in the soil and root architecture. Phosphorus contacts the root surface by three mechanisms: diffusion, mass flow and root interception. Majority of P supplied to plants comes from diffusion rather than mass flow due to the strong reaction of P with soil components. Plant roots also expand into the soil to acquire P. Mycorrhizal fungi that grow in association with roots also play a critical role in transferring P from soil to roots.
Crop Response to P Fertilizer Application
The probability of a crop responding to applied P can best be predicted by soil-test P levels. Recently, Culman et al. (2023) published a study comprising 457 field trials conducted over 45 years in Ohio to determine the crop response to applied P fertilizer. The response of corn, soybean, and wheat to P fertilizer application was evaluated in these trials across 40 Ohio counties. The field trial period ranged from one year to multiple years, including three long-term (16-year) P trials.
Soil P levels in the study ranged from 2.7 to 279 ppm across sites, with a median of 21.7 ppm. Currently, as per bulletin 974 Tri-State Fertilizer Recommendations for Corn, Soybean, Wheat, and Alfalfa, the critical level for corn and soybean is 20 ppm, and for wheat, it is 30 ppm, signifying a diverse set of soils—some likely and some not likely to respond to P fertilizer application (Culman et al., 2020). Across 457 field trials, P fertilizer application increased the grain yield in 107 trials (Fig. 3). On average, an increase of 22 bushels per acre in grain yield was observed at responsive sites. The response rate of crops also varied, with corn showing an increase in 29.9% of trials, soybean in 14.2% of trials, and wheat in 36.8% trials. Critical soil test levels ranged from 9 to 54 ppm based on various models, with an average value of 20 ppm across all crops and datasets.
Culman et al. (2023) classified the data into discrete categories based on relative yield and soil P levels rather than using a single critical value to explain yield outcomes. Relative yield referred to the percentage of yield obtained from unfertilized soil out of the maximum yield obtained across all treatments. In other words, a relative yield of 100% indicates no benefit of adding fertilizer to crop. The lower the value of relative yield, the higher the benefit of adding fertilizer to the crop. Relative yields were classified into five categories: <70%, 70%–80%, 80%–90%, 90%–95%, and 95%–100% and soil test values were classified as <10, 10–20, 20–30, 30–40, and >40 ppm.
As the P values decreased, relative yield decreased as well (Fig. 4). As an example, the median relative yield was 99.2% when the soil P levels were above 40 ppm. In contrast, the median relative yield dropped to 87% as the P dropped below 10 ppm. These results indicate that when P levels decrease, yield can be increased with P application.
Furthermore, the risk of yield loss with no P application decreased as the soil P level increased. As per the discrete classification, only 12%–14% of trials responded to P fertilizer in 20–30, 30–40, and > 40 ppm. As the P levels decreased, the response rate to P fertilizer application doubled at 10-20 ppm and increased to 66.7% at P levels <10 ppm (Table 1). In other words, the probability of a positive crop yield response to P application is much more likely in low-P soils (<20 ppm), and the crop yield response dramatically decreases as soil P levels increase above 20 ppm.
Apart from soil-test P levels, factors such as soil temperature, pH, compaction, and drainage can interact and impact the crop response to P fertilizer. Unlike nitrogen, P is immobile in soil. Any soil condition that inhibits root growth—and, therefore, the root’s capacity to intercept soil P—increases the probability of increasing the crop’s response to P application. For example, a poorly drained soil inhibits root proliferation. In such poorly drained soils, the crop will likely respond to P application. Another example is crop response to P application under acidic conditions. Banding of P fertilizer in acidic conditions often results in a crop response to P fertilizers, even if the soil P levels are in optimum range. The added P in acidic conditions reduces aluminum (Al) toxicity to crops and results in yield increases. However, it is worthwhile to invest in improving soil drainage or acidity issues to avoid P application to soils that are high in soil P. In other words, if soil P levels are in or above the optimum range but crops are P-deficient, it is critical to improve the physical and chemical soil conditions to reduce P application for economic and environmental reasons.
Phosphorus (P) is an essential plant nutrient required to optimize crop yields. Soil P is present in various forms, such as organic, inorganic, and soil-solution P. The majority of P uptake occurs from the soil-solution P pool and dissolution of secondary minerals. The crop response to P fertilizer application is generally governed by soil-test P levels. According to a recently published study, the majority of increased crop yield response was observed when P levels were below 20 ppm (Culman et al., 2023). The results indicated that the classification of soil test values into discrete categories could be more useful for end users to predict the crop response than a single critical soil test value. Overall, the probability of crop response and an incremental gain in grain yield from applied P increases as the soil-test P level decreases. Therefore, adding P fertilizer is less likely to show any yield benefits in a field with a soil P level of more than 20 ppm. Furthermore, if there is a crop response to P fertilizer in soils with more than 20 ppm of P, the amount of yield increase will be relatively less when compared to fertilizing in less than 20 ppm soil P conditions. The above study had a very large and diverse dataset, encompassing small plots and large strip trials over 40 Ohio counties, resulting in robust findings on phosphorus management.
Culman, S., Fulford, A., Camberato, J., & Steinke, K. (2020). Tri-state fertilizer recommendations for corn, soybean, wheat, and alfalfa (Bulletin 974). College of Food, Agricultural, and Environmental Sciences, The Ohio State University.
Culman, S., Fulford, A., LaBarge, G., Watters, H., Lindsey, L. E., Dorrance, A., & Deiss, L. (2023). Probability of crop response to phosphorus and potassium fertilizer: Lessons from 45 years of Ohio trials. Soil Science Society of America Journal, 87(5): 1207–1220.